A Study On Tectonic Geomorphology Of Namche Barwa And Activity Of The Faults

The eastern Himalayan syntaxis refers to the apex of the eastern part of the Himalayan arc-shaped mountains,located in the conjunction of the southeastern margin of the Tibetan Plateau,the northeastern Indian subcontinent and the northwest corner of Myanmar.It is the southern edge of the India-Eurasia collision,which began to uplift rapidly since the early Pleistocene.Since the collision,remarkable crustal shortening has taken place in this area.For example,in the west of the syntaxix,the Chayu island arc between the Tuting-Tiding suture and Nujiang suture was shortened by 200km.While in the west,east-west thrusts dominate the tectonic deformation,such as the Main Central Thrust?MCT?,Main Boundary Thrust?MBT?,Yarlung Tsangpo suture?IYS?and Renpo-Zedang Thrust?RZT?.During the collision between the Indian plate and Lhasa plate,with crustal shortening,three syntaxes in different directions developed successively which deformed towards south,i.e.the Namche Barwa syntaxis?NBS?,Sung syntaxis?SS?and Assam syntaxis?AS?.The Nache Barwa syntaxis is inside the big bend of Yarlung Zangbo River,as the front of the collision zone.The intensive compression made the Indian continent in the south indented into Eurasian continent to the north,where strongly dislocating and sharply bending of the Yarlung Zangbo suture zone occurred at the end of Eocene?37Ma?,leading to formation of a thumb-shaped structure.Afterwards,bounded by the left-slip fault in west and right-slip in east,the Nache Barwa metamorphic massif advanced to north and deeply plunged beneath the Gangdse granite?Lhasa massif?,forming the embryonic NBS.During 7-6Ma,gravitational collapse occurred when the Nache Barwa uplifted to the maximum height,with a series normal faulted in the periphery and interior.At 3Ma,In this region,the rate of uplift in Nache Barwa was higher than surrounding areas,which is considered to be the core of the Eastern Himalayan syntaxis?uplift center?.However,recent studies indicate that the earthquakes occur frequently at the top of Namche Barwa syntaxis include Linzhi M6.9,but seismic activity is relatively weak in internal area,while that in the southeastern region is strong where occurred the magnitude 8.6 earthquake in the Assam area in 1950.As a newly generated structure,the Assam syntaxis formed in13-3Ma,of which the tectonic features are controlled by the MBT and Main Front Thrust?MFT?.Similarly,GPS data show that the motion rate of the India plate in the AS is larger than that in the NBS.With weak tectonic movement,whether the uplift rate of NBS is also lower than AS?Has the eastern syntaxis?uplift center?shifted to the southeast?AS??South of Nache Barwa,due to well-developed pluvial fans and coverage of snow and vegetation,it is difficult to collect proper samples for thermal chronology.So far there is no detailed study on tectonic uplift of this area,lacking data for comparison with NBS,thus limiting understanding of recent dynamics of the eastern Himalayan syntaxis.The Motuo fault is located in the east of the NBS,adjacent to the Aniqiao fault which is the east boundary of Namche Barwa uplift.Some researchers suggested that the Motuo fault is the eastern boundary of the Aniqiao ductile shear zone,which is closely related with the tectonic movement of the Nache Barwa uplift.Many earthquakes greater than M3.0 during the period from 1974 to 2014 and aftershocks of the Ms8.0 earthquake of 1950 occurred in the vicinity of the Motuo fault,implying that tectonic activity around this fault is very intense since the Late Quaternary.However,as the Motuo fault lies in the Yarlung Tsngpo canyon,which has subtropical climate and heavy forests,as well as high-mountain and deep valleys where the surface erosion is very strong,there is no evidence for the spatial distribution,sliding properties and activity since Late Quaternary of the fault.The Milin fault is located on the west of NBS,obliquely crossing the Yarlung Zangbo suture zone,of which the activity is also controlled by the Namche Barwa uplift.In 1862,M6₄³ earthquake happened nearby the Millin fault,but no surface rupture found.Because of action from strong exterior forces that reformed the fault trace,or due to its broad width and influence of extensive lake strata,the surface expression of activity of the Millin fault is not obvious and not continuous,causing difficulty in tracing it.At present,there is no thorough study on its movement nature and activity since Late Quaternary.In the Nache Barwa area,the Yarlung Tsangpo River hosts abundant water resources,indicative of an ideal site of hydropower station construction.This region hosts,however,also a lot of active faults,among which the Motou and Millin faults have a large scale,where the potential sources of M8 earthquakes have been assessed in seismic risk zonation.Thus the research on the uplift of NBS and surroundings and activity of the Motuo and Millin faults is significant for understanding the tectonic evolution and dynamic mechanism of the Eastern Himalayan syntaxis as well as for the hydropower project on the reach of the Yarlung Tsangpo River in this region.In view of the natural environment limitation and the existence of scientific problems in the eastern Himalayan syntaxis region,this work makes a detailed study on the Eastern Himalayan syntaxis from the perspective of geomorphic development stage based on quantitative geomorphology and regional climate distribution using DEM data.The thesis describes the state of tectonic uplift-surface denudation of different regions,and further explores the regional dynamic mechanism.The characteristics of alluvial elements in the lower reaches of the Yarlung Zangbo River are studied in detail,and the ages of the terraces are measured by OSL which reveal the incision rate of the Yarlung Zangbo River in the region and its tectonic implications.Using remote sensing image interpretation,field geological investigations,exploration trenching and paleoearthquake analysis,combined with tectonic geomorphic parameters,this thesis presents a detailed study on the Motuo and Millin faults,exploring its formation mechanism and the dynamic background combined with geomorphology and tectonic movement of the Eastern Himalayan syntaxis.The main conclusions are as presented below.1.The relationship between the tectonic geomorphic parameters and the tectonic activity of the eastern Himalayan syntaxisThis issue is addressed by using the ASTER GDEM data of the horizontal resolution of 30m,and the risk of regional major earthquakes is preliminarily assessed.?1?The Namche Barwa area is in an erosive mature stage,the regional tectonic uplift and denudation are in a highly balanced status and the uplift of the this area is relatively slow at present.The geomorphology in the northwest is close to mature stage of landform evolution,but beginning to turn into the old age with a relatively slow uplift rate.The geomorphology of southeast and northeast of Assam is in a young stage of evolution,belonging to the erosion stage of infancy.The landform in northeast of Assam is closer to the early stage of infancy age.From the west of Namche Barwa to Assam syntaxis in southeast,the uplift rate increases gradually.Consequently,the core of the eastern Himalayan syntaxis?uplift center?moves towards to the southeast,and the new core position may be located in the northeast of Assam.?2?The fractal dimensions are high in some area of southeast of of Namche Barwa uplift and north of Assam,implied that the surface fluctuation frequency of thie area is very high result of strong tectonic uplift and erosion;The high fractal dimensions in Indian plain implied a high surface roughness caused by high-density river network;As the area of Namche Barwa uplift is erosed only by glacier with a low surface fluctuation frequency,the fractal dimensions is low although it has a significant topographic relief in the area;The fractal dimensions are low along the rivers,but not sensitive to the faults.?3?The counter-clockwise rotation of the Indian plate reflects that the northward motion rate of the NBS is relatively weakening.In combination with GPS data,it is suggested that the Nache Barwa metamorphic massif has weakened or stopped its subduction beneath the Lhasa massif,leading to relatively smaller compressive stress in the uplift area.In the northeast of Assam,the high-strength wedging and compression result in rapid uplift of this region at present.2.Features and age of terraces at the lower reaches of Yarlung Zangbo River and formation agesThis study made a detailed analysis of the characteristics of multiple terraces along the lower reaches of the Yarlung Zangbo River within Motuo County.?1?T1,T2,and T3 terraces alluvium sections outcrop intermittently along the lower reaches of the Yarlung Zangbo River within Motuo County.The terraces alluvium of near Linduo,Dexing village and Guoguotang comprise mainly poorly rounded coarse sands and rock fragments,no fine-grain component found,and the terrace T1 alluvium north of Dexing Village is made up of clay and silt.In light of composition features of terraces on the Yarlung Tsangpo River,the sedimentation process of terraces in this area is possibly related to the breaking of the paleo-barrier lake in the middle reaches.?2?The formation age of terrace alluvium in the north of Dexing Town is18.2±3.1kyr.In combination with its altitude with respect to the river,the Yarlung Zangbo River incision rate is 4.7mm/yr since 18.2ka in this section.There is still a drop of 430m between the section and the local erosion dtum,indicating that current tectonic uplift rate is relatively high in the east of the NBS.3.The movement characteristics and activity of Motuo faultThrough the analysis of DEM data,combined with remote sensing image interpretation and field trenching work,this thesis studies movement characteristics and the fault activity during late Quaternary of the Metok fault in detail.?1?The Motuo fault stretches on each side of the Yarlung Zangbo River,consisting of several obliquely stepped secondary faults which strike in 50°⁶0°.The length of the determined active fault is about 70 km.From sediments and dating data in the trenches,it was active during Holocene with the latest event 2800 years B.P.?2?The Motuo fault is dominated by left-lateral strike-slip,measured by GPS as a slip rate of 0.5±2.0mm/a,but this value has a major uncertainty.Overall this fault is a thrust with local nomal faulting.Since 18.2ka its vertical slip rate is greater than0.05mm/a.The maximum potential magnitude of earthquakes on the Motuo fault is Ms8.0.4.The movement characteristics and the activity of Millin faultThrough detailed field investigation and trench excavation work,the geometric distribution,nature of movement and the evidence of activity since the lateQuaternary of the Millin fault were obtained.Concluded as follow:?1?The Millin fault is distributed on the west side of the Namche Barwa syntaxis,consisting of two roughly parallel secondary faults.It starts from Lilong Town,crossing Millin County and Lulong,ending with Tongmai Town.Its strike converts from NE to nearly NS on the west side of the Gala White Peak and converts back to NE,where the fault crossing the west side of the Gala White Peak.?2?The latest movement of the northern segment of the Millin fault?nearly N-S?shows thrust movement along the dip direction.The faulted strata indicate that there have been obvious activities since the late Quaternary along the segment,the vertical slip rate is 0.17-0.27mm/a.According to the fault displacement-magnitude empirical formula,the maximum potential magnitude of the northern segment is evaluated as7.2.However,the tectonic landforms of the southern segment of the Milin fault?NE?are not obvious,and the lacustrine sediments and fluvial facies sediments in the fault area are not affected by the fault activity,it was active since the late Quaternary,and with a dextral strike slip rate of 0.4±2.0mm/a,with a major uncertainty.The maximum potential magnitude of the southern segment is evaluated as 7.7.?3?The anticlockwise rotation of the Indian plate against the Eurasian plate in the Namche Barwa syntaxis produced the tectonic compressive stress,which is the main dynamic mechanism of the thrust movement of the Millin fault.5.Faulting mechanism and massif movement model on either side of Namche Barwa syntaxisIndian plate pushed rapidly to the north in the area of Assam syntaxis than the Namche Barwa syntaxis metamorphic body,that is the main dynamic mechanism of left-lateral movement of the motuo fault.The Namche Barwa syntaxis metamorphic body pushed slowly to the north,and had even stopped subducted into the Lhasa block since the late Quaternary.As a result of the anticlockwise rotation of the Indian plate,the body began to push to the northwest?west?,and the crustal shortening of SE-NW caused by extrusion is absorbed by the thrust movement of the Motuo fault and the thrust and left-lateral movement of the northern segment of Miulin fault.6.Seismogenic structure of the Ms 6.9 LinZhi earthquakeThe seismogenic fault of Ms 6.9 LinZhi Earthquake is mainly characterized by high dip angle and dip NE.Its earthquake source area is located in the compressive stress field of NE-SW direction.The coseismic displacement is characterized by uplift to the NE side of the fault,descending to the SW side which indicates reverse fault movement.The LinZhi Earthquake is related to the high-angle and northeastern thrust movement at the top of the Namche Barwa syntaxis.